Systems and methods for closed loop dehydration of a mercury removal unit

ABSTRACT

Saturated mercury adsorbent in a gas mercury removal unit is dehydrated in a LNG, LPG or cryogenic gas plant using a regeneration gas stream. Spent regeneration gas stream is then condensed and the water is removed therefrom to form a renewed regeneration gas stream in a closed loop. The regeneration gas stream is compressed and recycled to a location in the plant upstream of an acid gas removal unit or upstream of a regeneration gas dehydration unit such that the regeneration gas stream is not sent to a flare. Conventional plants can be retrofitted to achieve improved process efficiencies, cost savings and environmental benefits.

FIELD

The present disclosure relates to the field of mercury removal unitsthat utilize adsorption beds to remove mercury from gas, and furtherrelates to systems and methods for dehydrating the adsorption beds insuch mercury removal units. The gas is then subjected to cryogeniccooling to form liquefied gas.

BACKGROUND

Mercury removal units utilizing adsorption beds are used in cryogenicgas plants, such as natural gas liquids (NGL) recovery and liquifiednatural gas (LNG) production plants, to remove mercury from a feed gas,e.g., natural gas. Mercury must be removed to prevent damage to aluminumheat exchangers. Mercury removal beds are dehydrated using hot, dry gas,which can be sourced from nitrogen or process gas downstream of amolecular sieve dehydration unit in the cryogenic gas plant, in whichwater is adsorbed onto molecular sieve material. The hot, dry gas flowsthrough the mercury removal bed to desorb water from the mercuryadsorbent material. The spent, wet gas from the mercury removal bed istypically sent to flare, rather than being recovered. This results insignificant costs associated with unrecovered nitrogen or process gas,as well as undesirable environmental impacts and potential financialpenalties associated with flaring.

It would be desirable to have a process for dehydrating mercury removalbeds resulting in cost savings and reduced environmental impact.

SUMMARY

In one aspect, a system is provided for dehydrating a mercury removalunit in a plant to produce liquefied natural gas, liquefied petroleumgas, and/or cryogenic gas. The system includes a mercury removal vesselcontaining mercury adsorbing material for adsorbing mercury from a feedgas stream contacting the mercury adsorbing material, thereby forming amercury depleted gas stream to be further processed in the plant. Themercury removal vessel in dehydration mode has a dry gas inlet and a wetgas outlet, and the mercury removal vessel in adsorption mode has a feedgas inlet and a mercury depleted gas outlet. A condenser is incommunication with the wet gas outlet of the mercury removal vessel indehydration mode for condensing water to form a stream containing waterand regeneration gas. A separator is provided for separating the waterand the regeneration gas from the stream containing water and gasthereby forming a water stream and a regeneration gas stream. Acompressor is provided for compressing the regeneration gas stream. Aconduit is provided for passing the regeneration gas stream from thecompressor to a location in the plant upstream of an acid gas removalunit or upstream of a regeneration gas dehydration vessel such that theregeneration gas stream is not sent to flare.

In one aspect, a method is provided for method for dehydrating a mercuryremoval unit in a plant to produce liquefied natural gas, liquefiedpetroleum gas, and/or cryogenic gas. The method includes contacting themercury adsorbing material with a regeneration gas stream therebydesorbing water from the mercury adsorbing material within the mercuryremoval vessel in dehydration mode to form a spent regeneration gasstream. The spent regeneration gas stream is condensed in a condenser toform a stream containing water and gas. The water and the gas from thestream are separated thereby forming a water stream and a regenerationgas stream. The regeneration gas stream is compressed in a compressor.Finally, the regeneration gas stream is recycled from the compressor toa location in the plant upstream of an acid gas removal unit or upstreamof a regeneration gas dehydration vessel such that the regeneration gasstream is not sent to flare.

DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims and accompanying drawings. The drawings arenot considered limiting of the scope of the appended claims. Theelements shown in the drawings are not necessarily to scale. Referencenumerals designate like or corresponding, but not necessarily identical,elements.

FIG. 1 is a simplified schematic diagram illustrating a processincluding a mercury removal unit according to the prior art.

FIGS. 2-3 are schematic diagrams illustrating processes includingmercury removal units according to exemplary embodiments.

DETAILED DESCRIPTION

Referring to FIG. 1, shown is a simplified schematic diagramillustrating a process including a mercury removal bed 2 in dehydrationmode according to the prior art. Mercury removal adsorbent material inbed 2 is dehydrated using dry gas 1, which is sourced from aregeneration gas source 3 which can be nitrogen or process gasdownstream of the plant's primary dehydration unit (not shown). The drygas is heated at heater 4 with temperature control to maintain safeoperating margin within the allowable limits of the mercury removaladsorbent material. The hot, dry gas 1 then flows through the mercuryremoval bed to desorb water from the adsorbent material. After passingthrough the mercury removal bed 2 and desorbing water, the spent, wetgas 5 is typically sent to the flare 6. This results in significantcosts associated with unrecovered nitrogen or process gas, andundesirable environmental impacts associated with flaring. Also shown isa flow path of regeneration gas from the plant's primary dehydrationunit (not shown), coming from the same regeneration gas source 3described above. In condenser 8, regeneration gas is condensed to form astream 9 containing water and gas. The water and the gas from the streamare separated in a separator 10 thereby forming a water stream 11 and aregeneration gas stream 12. The regeneration gas stream 12 is compressedin a compressor 13. The compressed regeneration gas stream 14 is sent toa location 16 in the plant upstream of the plant's primary dehydrationunit (not shown). The primary dehydration unit, also referred to as themain dehydration unit, includes at least two adsorbent (molecular sieve)bed containing vessels arranged in parallel.

Referring to FIG. 2, a system 100 and its operation for dehydrating amercury removal unit in a plant to produce liquefied natural gas,liquefied petroleum gas, and/or cryogenic gas will now be described. Themercury removal unit includes at least two adsorbent bed containingvessels arranged in parallel. Shown is a single mercury removal bed 2 indehydration mode, as described with respect to FIG. 1. In oneembodiment, the spent, wet gas 5 from the mercury removal bed 2 isdirected to the condenser 8 rather than to the flare 6, also referred toas “to flare 6.” The flow path of gas through the condenser 8, separator10 and compressor 13 and the flow path of gas through the heater 4 andthe mercury removal unit 2 are thus integrated, forming a closed loopfor dehydrating mercury removal beds, in which the spent, wet gas 5 isrecovered and recycled. Similar to the prior art, the mercury removalbed 2 is dehydrated using dry gas 1 that is sourced from nitrogen orprocess gas that is then heated. This hot, dry gas 1 flows through themercury removal bed 2 to desorb water from the adsorbent material. Afterpassing through the mercury removal bed 2 and desorbing water, thespent, wet gas 5 is recovered and recycled, rather than being sent toflare 6. The spent, wet regeneration gas 5 flows through piping from theoutlet of the mercury removal bed 2 to the regeneration gas condenser 8,in which the gas 5 is cooled to condense the bulk of the water therein.The condensed water is separated in the regeneration gas knock-out (KO)drum 10, also referred to as the separator 10, and then the gas iscompressed by the regeneration gas compressor 13. The compressed gas isrecycled back to a location 16 at the front end of the plant, upstreamof the primary dehydration unit. After flowing through the front end ofthe plant and dehydration unit, the dry process gas is again sourced asregeneration gas source 3 and heated using heater 4 to dehydrate themercury removal bed(s) 2. This heated gas 1 is again flowed through themercury removal bed(s) 2 to desorb water. The process continues untilthe mercury removal beds are sufficiently dehydrated. This closed loopprocess results in significant cost savings and minimizes theenvironmental impact of flaring.

In one embodiment, referring to FIG. 3, an alternative system 200 andits operation for dehydrating a mercury removal unit 2 in a plant toproduce liquefied natural gas, liquefied petroleum gas, and/or cryogenicgas will be described. In this embodiment, an independent, closed loopis provided for dehydrating the mercury removal beds, in which thespent, wet regeneration gas 5 is recycled and recovered. As in system100 shown in FIG. 2, the mercury removal beds 2 are dehydrated using drygas 1, which is sourced from nitrogen or process gas that is thenheated. This hot, dry gas flows through the mercury removal beds todesorb water from the adsorbent material. After passing through themercury removal beds and desorbing water, the spent, wet gas 5 isrecovered and recycled, rather than being sent to the flare 6. Thespent, wet regeneration gas 5 flows through piping from the outlet ofthe mercury removal bed 2 to the regeneration gas condenser 8, in whichthe gas is cooled to condense the bulk of the water. The condensed water9 is separated in the regeneration gas KO drum 10, and then theseparated gas 12 is compressed by the regeneration gas compressor 13.Rather than recycling the compressed gas to the front end of the plantas in the previously described embodiment, the compressed gas 14 is sentto a secondary molecular sieve dehydration unit 22 for further removingmoisture from the regeneration gas stream 14 that includes a pair ofvessels 22A and 22B arranged in parallel and containing molecular sievematerial. The pair of vessels 22A and 22B alternate between absorptionand regeneration modes. At any given time, one of the vessels 22A is inadsorption mode to desorb water from the process gas before it isrecycled back to the regeneration gas heater 4 to dehydrate the mercuryremoval bed 2. The other vessel 22B is in regeneration mode, in which aslip stream of dry process gas is taken from upstream or downstream ofthe regeneration gas heater 4, depending on whether the vessel is inheating or cooling, respectively. As shown in FIG. 3, vessel 22A is inadsorption mode and vessel 22B is in regeneration mode. Vessel 22A islocated in a conduit 23 between the compressor 13 and a locationupstream of the heater 4. Vessel 22B is located in a conduit 24 betweena location upstream of the heater 4 and a location upstream of thecondenser 8. Conduit 25 directs hot, dry process gas to vessel 22R inregeneration mode. After this dry gas flows through the dehydratorvessel 22R, it is tied in with the spent, wet gas 5 from the mercuryremoval beds 2 upstream of the regeneration gas condenser 8. Thisprovides the further benefit of closed loop operation that isindependent of the front end of the plant, e.g., a location 16,including the main dehydration unit.

Dehydration unit 22 is smaller than the main dehydration unit of theplant. This smaller dehydration unit 22 removes residual moisture toensure the gas is very dry, i.e., having a moisture content below 1 ppmby volume. This dry gas is then heated by the regeneration gas heater 4,and passes back through the mercury removal beds 2. This processcontinues until the mercury removal beds 2 are sufficiently dehydrated.This closed loop process results in significant cost savings andminimizes the environmental impact of flaring.

Since the gas is not recycled back to the front end of the plant butrather to the secondary dehydration unit 22, certain advantages can berealized. The plant throughput can be increased accordingly, and/or theplant can be designed for lower flow rates thus reducing equipmentsizing and capital expense.

The embodiments herein provide a process for recovering nitrogen orprocess gas that is used for dehydrating mercury removal beds, so thegas need not be flared. The flaring conventionally associated withdehydrating mercury removal beds occurs whenever the beds are reloaded,i.e., after turnaround, or whenever the beds are exposed to moisturebreakthrough from upstream dehydration units. The embodiments hereinlead to significant costs savings since the nitrogen or process gas doesnot have to be sourced or purchased, and penalties related to flaringcan be avoided.

In one embodiment, a conventionally designed plant to produce liquefiednatural gas, liquefied petroleum gas, and/or cryogenic gas can beretrofit to reduce costs and avoid flaring. The prior art system shownin FIG. 1 can be retrofit to the system 100 shown in FIG. 2 byinstalling a section of conduit indicated by reference numeral 5 toconnect the outlet of the mercury removal bed 2 in dehydration mode toan inlet of the regeneration gas condenser 8. A valve 15 can be providedin conduit 5. Optionally, the system can be further retrofit to thesystem 200 shown in FIG. 3 by installing the molecular sieve dehydrationunit 22 and associated piping (23, 24 and 25) and valves (26, 27, 28 and29).

It should be noted that only the components relevant to the disclosureare shown in the figures, and that many other components normally partof systems using mercury removal units are not shown for simplicity.

For the purposes of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that can vary depending upon thedesired properties sought to be obtained by the present invention. It isnoted that, as used in this specification and the appended claims, thesingular forms “a,” “an,” and “the,” include plural references unlessexpressly and unequivocally limited to one referent.

Unless otherwise specified, the recitation of a genus of elements,materials or other components, from which an individual component ormixture of components can be selected, is intended to include allpossible sub-generic combinations of the listed components and mixturesthereof. Also, “comprise,” “include” and its variants, are intended tobe non-limiting, such that recitation of items in a list is not to theexclusion of other like items that may also be useful in the materials,compositions, methods and systems of this invention.

This written description uses examples to disclose the invention,including the best mode, and to enable any person skilled in the art tomake and use the invention. The patentable scope is defined by theclaims, and can include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims if they have structural elements that do not differ from theliteral language of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims. All citations referred herein are expressly incorporatedherein by reference.

From the above description, those skilled in the art will perceiveimprovements, changes and modifications, which are intended to becovered by the appended claims.

What is claimed is:
 1. A method for dehydrating a mercury removal unit in a plant to produce liquefied natural gas, liquefied petroleum gas, and/or cryogenic gas, the method comprising: a) providing a mercury removal vessel containing mercury adsorbing material for removing mercury from a feed gas stream contacting the mercury adsorbing material thereby forming a mercury depleted gas stream to be further processed in the plant, wherein the mercury removal vessel in regeneration mode has a regeneration gas inlet and a regeneration gas outlet, and the mercury removal vessel in adsorption mode has a feed gas inlet and a mercury depleted gas outlet; b) contacting the mercury adsorbing material with a regeneration gas stream thereby desorbing water from the mercury adsorbing material within the mercury removal vessel in regeneration mode to form a spent regeneration gas stream; c) condensing the spent regeneration gas stream in a condenser to form a stream containing water and gas; d) separating the water and the gas from the stream thereby forming a water stream and a regeneration gas stream; e) compressing the regeneration gas stream in a compressor; f) recycling the regeneration gas stream from the compressor to a location in the plant upstream of an acid gas removal unit or upstream of a regeneration gas dehydration vessel such that the regeneration gas stream is not sent to flare; and g) dehydrating the compressed regeneration gas stream in a first dehydration unit comprising a pair of dehydration vessels containing molecular sieve material, wherein the pair of dehydration vessels is arranged in parallel, alternating between absorption and regeneration modes, wherein the dehydration vessel in absorption mode receives gas from a line downstream of the compressor and outputs gas to a line upstream of the mercury removal vessel in regeneration mode, and wherein the dehydration vessel in regeneration mode receives gas from the line upstream of the mercury removal vessel in regeneration mode and outputs gas to a line downstream of the mercury removal vessel in regeneration mode and upstream of the condenser.
 2. The method of claim 1 wherein the mercury removal unit comprises two mercury removal vessels arranged in parallel.
 3. The method of claim 1 wherein the regeneration gas stream is sourced from a second dehydration unit or a nitrogen gas supply.
 4. The method of claim 1 wherein the mercury removal vessel has two ends and each end has an opening that can act as a vessel inlet or a vessel outlet depending on a direction of fluid flow through the mercury removal vessel.
 5. The method of claim 1 further comprising continuing steps (b) through (f) until the desorbing of water from the mercury adsorbing material in regeneration mode is sufficiently complete.
 6. The method of claim 1 further comprising heating the regeneration gas to a temperature sufficient to desorb water from the mercury adsorbing material prior to step (b). 